Light-emitting diodes (LEDs) are expected to be used as a next-generation new light source in conventionally known lighting apparatuses including fluorescent lights and incandescent lamps because of their high efficiency and long life. Thus, research and development for LED light sources using the LEDs are proceeding. Along with this, development of light-emitting diode driver circuits for driving the LEDs is also proceeding.
Conventionally, as such a light-emitting diode driver circuit, a light-emitting diode driver apparatus as shown in FIG. 4 is proposed (Patent Literature (PTL) 1). FIG. 4 is a diagram showing a circuit configuration of a conventional light-emitting diode driver apparatus disclosed in PTL 1.
As shown in FIG. 4, a conventional light-emitting diode driver apparatus 100 is an LED driver circuit for turning ON an LED 200, and includes a rectifier circuit 110, a light-emitting diode driver semiconductor circuit 120, a smoothing capacitor 111, a choke coil 112, and a diode 113.
The rectifier circuit 110 is a bridge type full-wave rectifier circuit including four diodes, and two opposite ends are connected to an AC power source and the other two opposite ends are connected to the smoothing capacitor. An AC supply voltage from the AC power source is full-wave rectified by the rectifier circuit 110 and smoothed by the smoothing capacitor 111, generating a DC input voltage Vin.
An end of the choke coil 112 is connected to the high electrical potential side of the smoothing capacitor 111 and the other end of the choke coil 112 is connected to the anode side of the LED 200. Moreover, the cathode terminal of the diode 113 is connected to the high electrical potential side of the smoothing capacitor 111. The diode 113 is connected to the choke coil 112 and the LED 200 in parallel, and supplies back electromotive force generated in the choke coil 112 to the LED 200.
The light-emitting diode driver semiconductor circuit 120 is connected to the cathode terminal of the LED 200, and controls an LED block circuit including the choke coil 112, the diode 113, and the LED 200.
The light-emitting diode driver semiconductor circuit 120 includes a drain terminal 120D for receiving an output voltage from the LED 200, a ground/source terminal 120GS connected to a ground potential, and a VCC terminal (reference voltage terminal) 120Vcc for outputting a reference voltage Vcc. The light-emitting diode driver semiconductor circuit 120 further includes, in the circuit configuration, a switching device block 121 for controlling a current flowing to the LED 200, a control circuit 122 for controlling the switching device block 121 based on a voltage VJ of the switching device block 121, a drain current detection circuit 123 for detecting a current flowing in the switching device block 121, and a start/stop circuit 124 for controlling a start and a stop of operations of the switching device block 121. It is to be noted that a capacitor 114 is connected between the VCC terminal 120Vcc and the ground/source terminal 120GS of the light-emitting diode driver semiconductor circuit 120.
The switching device block 121 includes a junction FET 121a and a switching device 121b that is an N-type MOSFET connected in series to each other.
The control circuit 122 includes a regulator 122a for regulating the reference voltage Vcc at a constant value, an oscillator 122b for outputting a MAXDUTY signal and CLOCK, and an ON-time blanking pulse generator 122c for providing, to an AND circuit, pulses for setting time period in which the detection of the current is not performed. The control circuit 122 intermittently turns ON/OFF the switching device 121b at a predetermined oscillation frequency based on an output signal from the start/stop circuit 124 and an output signal from the drain current detection circuit 123. An end of the regulator 122a in the control circuit 122 is connected between the junction FET 121a and the switching device 121b, and the other end is connected to the VCC terminal 120Vcc. The regulator 122a receives the voltage VJ, regulates the reference voltage Vcc at a constant level, and provides the reference voltage Vcc to the VCC terminal 120Vcc.
The drain current detection circuit 123 is a comparator, which outputs a signal indicating High when the voltage VJ is higher than a detected reference voltage Vsn and outputs a signal indicating Low when the voltage VJ is lower than the detected reference voltage Vsn. The current flowing in the switching device 121b is detected by comparing an ON-voltage of the switching device 121b and the detected reference voltage Vsn of the drain current detection circuit 123.
The start/stop circuit 124 receives the reference voltage Vcc, and outputs a start signal (output signal indicating High) when the reference voltage Vcc is higher than or equal to a predetermined value, and outputs a stop signal (output signal indicating Low) when the reference voltage Vcc is lower than the predetermined value.
In the conventional light-emitting diode driver apparatus 100 configured as above, an ON/OFF control of the switching device 121b is performed as described above by the control circuit 122 in the light-emitting diode driver semiconductor circuit 120.
When the switching device 121b is ON, the input voltage Vin causes a current to flow in a direction from the choke coil 112 to the LED 200, and then to the light-emitting diode driver semiconductor circuit 120 to turn ON the LED 200. At this time, magnetic energy is accumulated in the choke coil 112 due to the current flowing in the choke coil 112.
Moreover, when the switching device 121b is OFF, the back electromotive force generated by the magnetic energy accumulated in the choke coil 112 causes a current to flow in a closed loop of the LED block circuit including the choke coil 112, the LED 200, and the diode 113 in a direction from the choke coil 112 to the LED 200, and then to the diode 113. As a result, the LED 200 is turned ON.
As described above, the conventional light-emitting diode driver apparatus 100 is capable of controlling the current flowing to the LED 200 at a constant current even when the input voltage varies.